This invention relates to microwave monolithic integrated circuits (MMIC), and more particularly, this invention relates to interfaces and carriers used for mounting MMIC chips such as used in millimeter wave modules (MMW).
Millimeter wave (MMW) modules are becoming more commonplace as increasing use is made of millimeter wave transceivers and similar millimeter wave devices. Often, these modules are used with various transceiver designs having different transmitter and receive circuits that make use of a number of different microwave monolithic integrated circuit (MMIC) chips or die. Many of the MMIC chips are formed from Galium Arsenide (GaAs). These chips are often attached on an alumina, i.e., aluminum oxide, or similar dielectric substrate. Because of the extreme tolerances and necessity for thermal matching, millimeter wave modules typically use an expensive coefficient of thermal expansion (CTE) matched housing material, such as copper tungsten (CuW) or aluminum silicon carbide (AlSiC) to mount Galium Arsenide MMIC chips and the alumina or similar dielectric substrates. CTE matching is required to prevent the MMIC chips and the substrates on which they are attached from cracking as the housing material shrinks and expands during extreme temperature variations.
Most millimeter wave Galium Arsenide MMIC dies or chips and the accompanying ceramic substrates have a Coefficient of Thermal Expansion (CTE) that is between about 4 and 7 ppm/deg. centigrade (in some instances, closer to between about 4 and 6 ppm/deg. centigrade). This has required the use of similarly matched housing materials, such as the copper tungsten or aluminum silicon carbide materials as a metal carrier (or base plate of a housing) for heat sinking and attachment. Unfortunately, the lower cost housings, such as formed from aluminum, copper and other similar metallic or other materials, have a very high coefficient of thermal expansion, greater than about 16 ppm/deg. Centigrade, and therefore, cannot be used. The use of CTE matched carrier material has been required in the past to prevent the chips and substrates from cracking as the carrier material shrinks and expands versus temperature.
In some prior art modules, an epoxy preform is used to attach a ceramic substrate having MMIC chips to a carrier. Compliant epoxy has been used extensively in the past to attach CTE mismatched materials. The compliant characteristics of the epoxy allows it to have some elasticity, which enables the mismatched materials to expand at different rates without being separated. However, every compliant epoxy has a limited amount of elasticity, which limits the size of the bonded material to a few mils. The size of the bonding area has been restricted by the amount of CTE and mismatch. For example, the higher the CTE mismatch, the smaller is an allowable compliant epoxy bonding area.
An example of using non-CTE matched material that is less in cost is disclosed in the incorporated by reference Ser. No. 09/933,128 patent application and provides a unique structure and method to interface a housing and substrate material, both having a different coefficient of thermal expansion, without damaging or impacting the performance of the MMIC chip, RF interconnects and other material components. The millimeter wave (MMW) module for the microwave monolithic integrated circuit (MMIC) includes a carrier board formed of a dielectric material and having at least one MMIC die (chip) mounted thereon, and at least one interface line. A base plate is formed of a material that has a higher, unmatched coefficient thermal expansion (CTE) than the carrier board. The base plate supports the carrier board.
A housing is mounted over the carrier board and engages the base plate. This housing has at least one waveguide or subminiature coaxial connector (SMA) interface mounted thereon. A flexible circuit interconnect connects the subminiature coaxial connector(s) and the MMIC die through the interface line. A thermal interface member is positioned between the carrier board and base plate to aid in heat transfer between the base plate and housing and the lower CTE carrier board.
The flexible circuit interconnect could be formed as one of fuzz buttons or spring loaded self-adjusting interconnects, including the use of modified forms of pogo pins and similar spring segments and resilient members. The carrier board preferably, but not necessarily, comprises a plurality of layers of low temperature transfer tape (LTTT) to form a multilayer substrate board. The base plate and housing are formed from a material such as aluminum and/or similar metallic material. The thermal interface member comprises a heat transfer gasket that is formed from one of at least a phase change material, thermally conductive elastomer, or thermally conductive paste. Fasteners can secure the base plate and housing together.
Although this type of assembly solves the interface problem to allow the floating of a ceramic or other board material relative to a housing, including a base plate or carrier, there are still improvements that can be made by applying a MMIC assembly onto a CTE matched carrier. Some prior art techniques use an alumina substrate where Galium arsenide or other MMIC chips are attached directly to the CTE matched material either with solder or with silver epoxy. To provide efficient cooling for the MMIC chips, holes are cut through the ceramic and raised pedestals on the CTE matched carrier are used to directly attach the MMIC chips. The pedestals are also used to maintain the mounting surface of the MMIC chips at approximately the same level as the top portion of the ceramic board where all the radio frequency (RF) circuits and microstrip lines are present. In many chip and wire applications, co-planarity of the chip surface and the microstrip line is critical for radio frequency performance. Although the pedestals work well, the machining used to raise these areas is expensive because tight tolerances are required for precisely inserting the pedestals into cut holes on the ceramic board and provide a flat surface for mounting the chips.
It would be advantageous to replace a CTE matched carrier with a non-CTE matched carrier that is less expensive without having cracking and carrier material shrinkage or expansion versus temperature while also allowing the use of a large carrier.
The present invention advantageously sections a carrier into smaller subcarriers and eliminates the size limitation. In effect, the carrier has no size limitation. The present invention provides a microwave monolithic integrated circuit (MMIC) assembly where a dielectric substrate such as formed from alumina or other ceramic material has a surface on which radio frequency circuits and microstrip lines are formed. At least one MMIC chip opening is dimensioned for receiving therethrough a MMIC chip. A metallic carrier having a mismatched coefficient of thermal expansion to the dielectric substrate includes a component side or surface, which is secured to the dielectric substrate on the side (surface) opposing the radio frequency circuits and microstrip lines. The metallic carrier has at least one raised pedestal on the component side that is positioned at the MMIC chip opening. A MMIC chip secured on the pedestal and extends through the MMIC chip opening for connection to the radio frequency circuits and microstrip lines. Stress relief portions are formed in the metallic carrier that segment the carrier into subcarriers and provides stress relief during expansion and contraction created by temperature changes.
In one aspect of the present invention, the MMIC chip includes a circuit connection surface wherein the pedestal is dimensioned such that the circuit connection surface of the MMIC chip is positioned coplanar with the radio frequency circuits and microstrip lines on the dielectric substrate. The stress relief portions can be formed as grooves within the side of the metallic carrier opposite the component side or formed as cuts that extend through the carrier.
In yet another aspect of the present invention, the metallic carrier is formed substantially from copper or aluminum. The metallic carrier has a coefficient of thermal expansion between about 16 and about 17 ppm/degree centigrade and the MMIC chip and dielectric substrate has a coefficient of thermal expansion between about 6 and about 7 ppm/degree centigrade. An adhesive is positioned on those areas corresponding to subcarriers for adhesively securing the substrate to the carrier. In one aspect of the invention, the adhesive comprises a compliant epoxy.
In yet another aspect of the present invention, the stress relief portions comprise etched portions in which the metallic carrier has been removed. The subcarriers can be formed by etching the metallic carrier.
A method aspect of the invention allows the interfacing of a ceramic substrate, at least one microwave monolithic integrated circuit (MMIC) and metallic carrier having a coefficient of thermal expansion (CTE) that is not matched with a ceramic substrate and a MMIC. The method comprises the steps of segmenting the carrier with stress relief portions to form subcarriers and bonding the carrier with the ceramic substrate by an adhesive positioned at the subcarriers such that the stress relief portions and formed subcarriers provide stress relief during expansion and contraction created by temperature changes.
In yet another aspect of the present invention, the step of segmenting the carrier comprises the step of etching the carrier to form the stress relief portions. The segmenting can be accomplished by forming grooves on the carrier or by forming cut lines that extend through the carrier for segmenting the carrier into subcarriers.